U.S. patent application number 09/804104 was filed with the patent office on 2002-09-12 for time division multiplexing over broadband modulation method and apparatus.
Invention is credited to Johnson, Robert Edward Lee, Leatherbury, Ryan M..
Application Number | 20020126685 09/804104 |
Document ID | / |
Family ID | 25188191 |
Filed Date | 2002-09-12 |
United States Patent
Application |
20020126685 |
Kind Code |
A1 |
Leatherbury, Ryan M. ; et
al. |
September 12, 2002 |
Time division multiplexing over broadband modulation method and
apparatus
Abstract
A packet switch router that processes downstream digital
information to provide dedicated bandwidth to each subscriber
destination in a hybrid fiber coax (HFC) network. The router
includes a network module that terminates a network connection, a
switch that forwards data from the network module, and a channel
module. The channel module includes a switch interface, a cell
processing engine, one or more modulators, and a radio frequency
(RF) transmitter network. The switch interface forwards packetized
data from the switch to the cell processing engine. The cell
processing engine organizes the packetized data into multiple data
streams, encapsulates data in each data stream into data cells, and
multiplexes the data cells into a multiplexed cell stream. Each
modulator is configured to modulate a multiplexed cell stream into
an analog signal. The RF transmitter network up converts and
combines a plurality of analog signals into a combined electrical
signal for transmission.
Inventors: |
Leatherbury, Ryan M.;
(Austin, TX) ; Johnson, Robert Edward Lee;
(Austin, TX) |
Correspondence
Address: |
TREVOR Q. CODDINGTON
INTELLECTUAL PROPERTY DEPARTMENT
BROBECK, PHLEGER & HARRISON LLP
1333 H STREET, N.W.-SUITE 800
WASHINGTON
DC
20005
US
|
Family ID: |
25188191 |
Appl. No.: |
09/804104 |
Filed: |
March 12, 2001 |
Current U.S.
Class: |
370/432 ;
370/480 |
Current CPC
Class: |
H04L 2012/5605 20130101;
H04N 21/6168 20130101; H04Q 11/0478 20130101; H04L 2012/5606
20130101; H04N 21/6118 20130101; H04L 2012/561 20130101 |
Class at
Publication: |
370/432 ;
370/480 |
International
Class: |
H04L 012/56 |
Claims
1. A method of processing digital information by a point of
distribution to provide dedicated bandwidth to a plurality of
subscriber destinations via an HFC network, comprising: forwarding
data packets into a plurality of data streams, each data stream
corresponding to one of the plurality of subscriber destinations;
framing each data packet in each data stream; segmenting
encapsulated data packets into data segments; encapsulating data
segments of each of the plurality of data streams into data cells
to form a corresponding one of a plurality of cell streams;
multiplexing the plurality of cell streams into a multiplexed cell
stream; and modulating the multiplexed cell stream into a modulated
signal within a frequency channel.
2. The method of claim 1, prior to said forwarding, further
comprising: receiving digital information; and processing the
digital information into data packet information.
3. The method of claim 1, wherein said framing each data packet
comprises appending a packet header including a length value
indicative of the size of the data packet.
4. The method of claim 3, wherein said segmenting includes
incorporating the packet header in a first segment for each
segmented data packet.
5. The method of claim 4, wherein said encapsulating data segments
comprises appending a cell header to each data segment, the cell
header including an offset value indicating a beginning of a next
segmented data packet in the multiplexed cell stream.
6. The method of claim 5, further comprising: verifying that each
offset value is compatible with a length value for a corresponding
segmented data packet.
7. The method of claim 5, wherein said cell header includes a
synchronization value.
8. The method of claim 5, wherein said encapsulating data segments
further comprises: padding incomplete data cells with null values
to achieve equal-sized data cells in the multiplexed cell
stream.
9. The method of claim 1, wherein said multiplexing the plurality
of cell streams comprises: inserting data cells from each of the
plurality of cell streams into the multiplexed cell stream in a
round-robin manner.
10. The method of claim 1, wherein said multiplexing the plurality
of cell streams comprises: channelizing the multiplexed cell stream
into cell groups, each cell group having an equal number of time
slots; and inserting data cells from each of the plurality of cell
streams into the time slots of each cell group.
11. The method of claim 10, further comprising: assigning at least
one transport channel to each data stream, each transport channel
comprising a series of corresponding time slots; and said inserting
comprising inserting data cells from each of the plurality of cell
streams into corresponding time slots of assigned transport
channels.
12. The method of claim 1, further comprising: sending the
multiplexed cell stream as a synchronous cell stream to a
modulator.
13. The method of claim 12, further comprising: inserting null
cells to maintain a continuous synchronized cell stream.
14. The method of claim 12, further comprising: inserting a delay
between each data cell of the multiplexed cell stream.
15. The method of claim 1, wherein said encapsulating data segments
includes appending a cell header to each data cell, the cell header
including a synchronization value.
16. The method of claim 15, wherein said synchronization value is
in accordance with MPEG-2 format.
17. The method of claim 15, after said multiplexing and prior to
said modulating, further comprising: modifying periodic
synchronization values within cell headers that are appended to
each data cell; scrambling a payload of each data cell within the
multiplexed cell stream; and encoding data cells in the multiplexed
cell stream.
18. The method of claim 17, wherein said encoding is according to
the Reed-Solomon encoding scheme.
19. The method of claim 1, wherein said modulating is according to
quadrature amplitude modulation (QAM).
20. The method of claim 19, wherein said modulating is according to
QAM-256 modulation.
21. The method of claim 1, further comprising: said multiplexing
comprising multiplexing the plurality of cell streams into a
plurality of multiplexed cell streams; modulating each multiplexed
cell stream into a corresponding modulated signal within a
corresponding one of a plurality of frequency channels; and
combining the plurality of frequency channels into a single
electrical signal.
22. The method of claim 21, further comprising: converting the
electrical signal into an optical signal for transmission to an
optical node.
23. A method of providing dedicated bandwidth to each of a
plurality of subscriber destinations for delivering source
information over an HFC network, comprising: forwarding digital
information into a plurality of data streams, each data stream
corresponding to one of the plurality of subscriber destinations;
encapsulating the digital information in each data stream into data
cells; multiplexing the data cells of each of the plurality of data
streams into a multiplexed cell stream; modulating the multiplexed
cell stream into an analog signal in a frequency channel;
converting the analog signal to an optical signal; and transmitting
the optical signal to the plurality of subscriber destinations over
an HFC network.
24. The method of claim 23, prior to said forwarding digital
information, further comprising: receiving data packets at a
distribution hub; decapsulating the data packets to obtain packet
data in an original packet format; and re-assembling the packet
data into packets of the original packet format.
25. The method of claim 24, wherein the original packet format is
an IP packet format.
26. The method of claim 24, prior to said receiving data packets,
comprising: receiving an optical signal from a headend; and
converting the optical signal into the data packets.
27. The method of claim 23, further comprising: said forwarding
digital information including determining digital addresses
associated with the plurality of subscriber destinations.
28. The method of claim 23, prior to said forwarding, further
comprising: converting the digital information into data
packets.
29. The method of claim 28, wherein said encapsulating further
comprises: segmenting the data packets in each data stream into
packet segments; framing the packet segments with frame headers;
and encapsulating framed packet segments into the data cells, each
data cell including a cell header.
30. The method of claim 29, further comprising: said framing
including appending a frame header with a length value indicative
of the length of a data packet; and said encapsulating including
appending a cell header with a pointer value indicative of the
start of an encapsulated data packet.
31. The method of claim 30, further comprising: verifying a pointer
value with a length value to ensure data integrity.
32. The method of claim 30, wherein said encapsulating includes
inserting a synchronization value according to MPEG-2 format in the
cell header.
33. The method of claim 23, wherein said multiplexing further
comprises: inserting data cells from each of the plurality of data
streams in a round-robin manner to form the multiplexed cell
stream.
34. The method of claim 23, wherein said multiplexing further
comprises: assigning each of the plurality of data streams to at
least one of a predetermined number of transport channels of the
multiplexed cell stream; and inserting data cells from the
plurality of data streams into assigned transport channels of the
multiplexed cell stream.
35. The method of claim 23, prior to said modulating the
multiplexed cell stream, further comprising: encoding each data
cell of the multiplexed cell stream.
36. The method of claim 35, wherein said encoding comprises
encoding each data cell according to a Reed-Solomon encoding
scheme.
37. The method of claim 35, prior to said encoding each data cell
of the multiplexed cell stream, further comprising: scrambling each
data cell of the multiplexed cell stream.
38. The method of claim 23, wherein said modulating comprises
modulating the multiplexed cell stream according to quadrature
amplitude modulation (QAM).
39. The method of claim 23, further comprising: said multiplexing
comprising multiplexing the data cells of the multiple data streams
into a plurality of multiplexed cell streams; said modulating
comprising modulating each of the plurality of multiplexed cell
streams into a corresponding plurality of analog signals; up
converting each of the plurality of analog signals into a
corresponding one of a plurality of frequency channels; and
combining the plurality of frequency channels into an electrical
signal.
40. The method of claim 39, further comprising: converting the
electrical signal to an optical signal.
41. The method of claim 23, wherein said transmitting further
comprises: transmitting the optical signal to an optical node;
converting, by the optical node, the optical signal to an
electrical signal; and transmitting, by the optical node, the
electrical signal to the plurality of subscriber destinations via a
coaxial cable.
42. A channel module that processes downstream digital information
at a point of distribution to provide dedicated bandwidth for each
of a plurality of subscriber destinations in an HFC network,
comprising: an interface that receives packetized data; a cell
processing engine, coupled to the interface, comprising: a switch
that forwards the packetized data into a plurality of data streams;
an encapsulator that encapsulates the packetized data in each data
stream into data cells; and a channelizer that multiplexes the data
cells of the plurality of data streams into a multiplexed stream of
data cells; a modulator, coupled to the cell processing engine,
that modulates the multiplexed stream of data cells into an analog
signal; and a radio frequency (RF) transmitter network that up
converts the analog signal into a frequency channel.
43. The channel module of claim 42, wherein the cell processing
engine further comprises: a frame processor, coupled to the
interface and the switch, that decapsulates the packetized data and
re-assembles IP packets.
44. The channel module of claim 43, wherein the encapsulator
further comprises: a packet adaptation procedure (PAP) processor,
coupled to the switch, that frames IP packets in each data stream
with a frame header which includes a length value indicative of the
size of each IP packet.
45. The channel module of claim 44, wherein the encapsulator
further comprises: a cell convergence procedure (CCP) processor,
coupled to the PAP processor and the channelizer, that generates
the data cells by segmenting framed IP packets and encapsulating
each segment with a CCP header that includes a pointer value
indicative of the location of a next frame header in a stream of
data cells.
46. The channel module of claim 45, wherein the CCP processor pads
partial segments with at least one null value to create equal-sized
data cells.
47. The channel module of claim 46, wherein the CCP processor
further generates null data cells if input packetized data is not
available.
48. The channel module of claim 45, wherein the CCP processor adds
a synchronization value in accordance with MPEG-2 to the CCP
header.
49. The channel module of claim 42, wherein the channelizer
organizes the multiplexed stream of data cells into cell groups,
each cell group including a plurality of time slots.
50. The channel module of claim 49, wherein the channelizer inserts
data cells from each of the plurality of data streams according to
assigned time slots.
51. The channel module of claim 50, further comprising: a memory,
coupled to the channelizer, that stores a lookup table that maps
each time slot to an address of a corresponding subscriber
destination.
52. The channel module of claim 42, wherein the cell processing
engine inserts a delay between each data cell of the multiplexed
stream of data cells while transmitting to the modulator.
53. The channel module of claim 42, wherein the modulator further
comprises: a randomizer; an encoder; and a quadrature amplitude
modulator (QAM).
54. The channel module of claim 53, wherein the encoder comprises a
Reed-Solomon encoder.
55. The channel module of claim 53, wherein the QAM performs
QAM-256 modulation.
56. The channel module of claim 42, further comprising: the cell
processing engine providing a plurality of multiplexed data cell
streams; a plurality of modulators, each receiving a corresponding
one of the plurality of multiplexed data cell streams; and the RF
transmitter network including a combiner that combines a plurality
of frequency channels into a single electrical signal.
57. A packet switch router channel module that processes downstream
digital information at a point of distribution to provide dedicated
bandwidth for each of a plurality of subscriber destinations in an
HFC network, comprising: a network interface module that terminates
a network connection; a switch, that forwards packetized data from
the network interface module; and at least one channel module,
coupled to the switch, comprising: a switch interface that receives
packetized data from the switch; a cell processing engine, coupled
to the switch interface, that forwards the packetized data into a
plurality of data streams, that encapsulates the packetized data in
each data stream into data cells, and that multiplexes the data
cells of the plurality of data streams into at least one
multiplexed stream of data cells; a plurality of modulators, each
coupled to the cell processing engine and each configured to
modulate a corresponding multiplexed stream of data cells into an
analog signal; and a radio frequency (RF) transmitter network,
coupled to plurality of modulators, that up converts and combines a
plurality of analog signals into a combined electrical signal for
transmission.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to information delivery and
distribution, and more particularly, to a time division
multiplexing over broadband modulation method and apparatus that
enables the delivery of allocated, unshared and deterministic
bandwidth to subscribers in a network.
DESCRIPTION OF RELATED ART
[0002] The demand for broadband content by business and residential
subscribers is continually increasing. Broadband content includes
multiple types of entertainment programming, communications and
data, such as broadcast television channels, video on demand,
streaming video, multimedia data, internet access, voice-over-IP,
etc. To meet the increasing demand, it is necessary to increase
bandwidth to each subscriber and to improve quality of service.
Current delivery technologies include several variations of DSL
(digital subscriber line) technology, such as ADSL (asymmetrical
DSL) and the like, which uses telephony technology, cable modem
systems using television technology and HFC (hybrid fiber coax)
distribution networks, 2-way wireless local loop (WLL), including
2-way satellite, etc. The existing legacy technology for providing
broadband content is becoming increasingly inadequate to meet the
demand.
[0003] DSL technology is a method of delivering data over a twisted
pair of copper wires or twisted pair cables, and typically uses the
Public Switched Telephone Networks (PSTN). There are several major
problems with provisioning video services over the existing PSTN
and twisted pair cables (network plant). The existing network plant
is not uniform and most of the plant is old with poor copper
conditions that cause signal loss and line noise. In fact, ADSL
cannot be provisioned for a large portion of the population over
the existing plant because of significant distances to the closest
DSL Access Multiplexor (DSLAM) and poor conditions of the existing
plants. In addition, ADSL currently has a limited downstream
bandwidth, and inherently provides a very limited return bandwidth.
ADSL is not adequate for many types of content originating at a
subscriber destination, such as video conferencing and the like
because of its bandwidth limitations and characteristics.
[0004] Cable modem systems for delivery If data services using
Data-Over-Cable Service Interface Specifications (DOCSIS) utilize
the television broadcast spectrum and television technology to
broadcast so-called broadband data to subscribers. One problem with
delivery of broadband data (video on demand, streaming video, etc.)
using existing HFC networks is the limitation on available delivery
spectrum. The television data delivery systems have been
established to deliver data to subscribers over a television
broadcast spectrum extending from approximately 15 Megahertz (MHz)
to approximately 860 MHz. Delivery of analog television downstream
to the subscriber occupies the spectrum between approximately 54
MHz to 550 MHz, which leaves a relatively small range of spectrum
for the delivery of digital information over HFC cable modem
systems. The diplex filter separating the downstream from the
upstream is located within the frequency range of approximately 42
to 54 MHz in an extended sub-split frequency plan, which is common
for most consumer-based HFC systems. Therefore, the two effective
delivery frequency ranges using typical consumer-based HFC systems
are those between approximately 15-42 MHz (upstream) and those
between approximately 550-860 MHz (downstream).
[0005] Data-Over-Cable Service Interface Specifications (DOCSIS) is
a standard that specifies the methodology for delivering data
services over an HFC plant. DOCSIS defines a Cable Modem
Termination System (CMTS), which is an entity used to deliver data
services over an HFC network from a central distribution point.
These legacy systems use a shared frequency channel to broadcast
all data to every downstream subscriber. The shared channel is
generally 6 MHz wide providing a total data bandwidth of
approximately 27-38 megabits per second (Mbps) for digital
information. The channel, however, is shared among many
subscribers, so that the data rate varies dramatically depending
upon the time of use and the number of subscribers simultaneously
logged on. The quality of service is particularly low during
popular usage time periods. An exemplary legacy system might
distribute the shared channel among 4 separate nodes, each serving
approximately 500 subscribers or more, so that resulting downstream
data rate is often relatively low. The upstream shared channel is
usually smaller, such as 3.2 MHz or less, and a "poll and grant"
system is employed to identify data for upstream transmission. The
resulting upstream performance is often no higher (and sometimes
less) than a standard 56 Kbps modem.
[0006] It is desired to provide a system and method for
distributing information via existing and future communication
networks that meets the increasing demand for broadband
content.
SUMMARY OF THE INVENTION
[0007] A packet switch router according to embodiments of the
present invention processes downstream digital information at a
point of distribution to provide dedicated bandwidth for each of a
plurality of subscriber destinations in a hybrid fiber coax (HFC)
network. The packet switch router includes a network interface
module that terminates a network connection, a switch that forwards
packetized data from the network interface module, and at least one
channel module. The channel module includes a switch interface, a
cell processing engine, one or more modulators, and a radio
frequency (RF) transmitter network. The switch interface receives
and forwards packetized data from the switch to the cell processing
engine. The cell processing engine forwards the packetized data
into multiple data streams, encapsulates the packetized data in
each data stream into data cells, and multiplexes the data cells of
each the data streams into a multiplexed stream of data cells. Each
modulator is configured to modulate a corresponding multiplexed
stream of data cells into an analog signal. The RF transmitter
network upconverts and combines a plurality of analog signals into
a combined electrical signal for transmission.
[0008] A channel module in accordance with embodiments of the
present invention includes an interface that receives packetized
data, a cell processing engine, a modulator, and an RF transmitter
network. The cell processing engine includes a switch that forwards
the packetized data into one or more data streams, an encapsulator
that encapsulates the packetized data in each data stream into data
cells, and a channelizer that multiplexes the data cells of each
data streams into a multiplexed stream of data cells. In one
embodiment, the cell processing engine includes a frame processor
that decapsulates the packetized data in one format and
re-assembles packets into a different format. For example, the
packetized data may be re-assembled back into IP packets. The cell
processing engine may further include a packet adaptation procedure
(PAP) processor that frames the re-assembled packets in each data
stream with a frame header including a length value indicative of
the size of each packet. The encapsulator may further include a
cell convergence procedure (CCP) processor that generates the data
cells by segmenting framed packets and encapsulating each segment
with a CCP header. The CCP header includes a pointer value
indicative of the location of a next frame header in a stream of
data cells. In a particular embodiment, the CCP processor adds a
synchronization value in accordance with MPEG-2 to spoof an MPEG
data stream. The CCP processor may be configured to pad partial
segments with at least one null value to create equal-sized data
cells. The CCP processor may further be configured to generate null
data cells if input packetized data is not available to maintain a
continuous synchronous data stream.
[0009] In more particular embodiments, the channelizer operates to
organize the multiplexed stream of data cells into cell groups,
where each cell group includes multiple time slots. The channelizer
inserts data cells from each of data stream according to assigned
time slots. A memory may be included, which stores a lookup table
with time slot assignments for each data stream. In a particular
embodiment, the lookup table maps timeslots to destination IP
addresses corresponding to each data stream, where the destination
IP addresses each correspond to a subscriber destination. The
modulator may include an encoder or the like that adds redundant
data to each data cell prior to transmission to enable the receiver
to reconstruct data cells in the event of lost or erroneous data.
In such configuration, the cell processing engine may be configured
to insert a delay between each data cell of the multiplexed stream
of data cells while transmitting to the modulator to maintain
timing between the cell processing engine and the modulator. In one
embodiment, the modulator includes a randomizer, an encoder, and a
quadrature amplitude modulator (QAM). A QAM-256 modulator is
contemplated to achieve high data throughput in the downstream
direction. The encoder may be a Reed-Solomon encoder or the like.
Several multiplexed data cell streams are contemplated depending
upon the particular data throughput that is desired. In multiple
data stream configurations, the cell processing engine outputs more
than one multiplexed data cell stream, each provided to a
corresponding modulator. The RF transmitter network includes a
combiner that combines multiple frequency channels into a single
electrical signal.
[0010] It is appreciated that each data stream may correspond to
one of multiple downstream subscriber destinations. The process of
converting each data stream into a stream of cells enables
multiplexing the cells from multiple data streams. This results in
a single multiplexed data stream that is used to service multiple
subscribers. Furthermore, dividing the stream into cell groups,
each group having a fixed number of time slots or transport
channels, enables each subscriber to have a dedicated downstream
bandwidth. For example, in a particular embodiment employing 6 MHz
channels and QAM-256 modulation, each frequency channel is capable
of supporting approximately 40 Mbps data throughput. Time division
multiplexing or time slot channelization of the frequency channel
allows the 40 Mbps throughput to be further sub-divided. For
example, organizing the cell stream into eight different transport
channels allows each transport channel to support approximately 5
Mbps. Thus, eight different subscriber destinations may each be
allocated a dedicated channel having 5 Mbps bandwidth. Of course, a
given subscriber destination may be allocated multiple time slots
to achieve an incremental increase in the dedicated bandwidth to
that subscriber. For example, 3 of 8 transport channels assigned to
a single subscriber destination provides approximately 15 Mbps to
that subscriber destination.
[0011] A method of processing digital information by a point of
distribution in accordance with embodiments of the present
invention provides dedicated bandwidth to multiple subscriber
destinations via an HFC network. The method includes forwarding
data packets into multiple data streams, framing each data packet
in each data stream, segmenting encapsulated data packets into data
segments, encapsulating data segments of each data stream into data
cells to form a corresponding cell streams, multiplexing the cell
streams into a multiplexed cell stream, and modulating the
multiplexed cell stream into a modulated signal within a frequency
channel. The method may further include receiving and processing
digital information into data packet information. The method may
further include assembling the data packet information into data
packets.
[0012] The framing may include appending a packet header including
a length value indicative of the size of the data packet. The
segmenting may include incorporating the packet header in a first
segment for each segmented data packet. The encapsulating data
segments may include appending a cell header to each data segment,
where the cell header includes an offset value indicating a
beginning of a next segmented data packet in the multiplexed cell
stream. The encapsulating may include adding a synchronization
value in accordance with the MPEG-2 format, which is particularly
advantageous in that off-the-shelf components may be used to reduce
cost and development time. The method may further include verifying
that each offset value is compatible with a length value for a
corresponding segmented data packet. The cell header may include a
synchronization value to enable synchronization with the downstream
subscriber destination equipment. The encapsulating may further
include padding incomplete data cells with null values to achieve
equal-sized data cells in the multiplexed cell stream. The
multiplexing may include inserting data cells from each cell stream
into the multiplexed cell stream in a round-robin manner.
[0013] In a more particular embodiment, the multiplexing may
include organizing the multiplexed cell stream into cell groups,
where each cell group has an equal number of time slots, and
inserting data cells from each cell stream into the time slots of
each cell group. The method may further include assigning at least
one time slot of the cell group to each data stream, and inserting
data cells from each cell stream into assigned time slots. The
method may further include sending the multiplexed cell stream as a
synchronous cell stream to a modulator.
[0014] After multiplexing and before modulating, the method may
include modifying periodic synchronization values within cell
headers that are appended to each data cell, scrambling a payload
of each data cell within the multiplexed cell stream, and encoding
data cells in the multiplexed cell stream. The encoding may be
according to any suitable encoding scheme, such as according to
Reed-Solomon or the like. The modulation may be according to any
known or later developed modulation techniques, such as quadrature
amplitude modulation (QAM) or the like as previously described.
[0015] The multiplexing may include multiplexing the cell streams
into multiple cell streams, each multiplexed in a similar manner.
Modulating is performed on each multiplexed cell stream to achieve
a corresponding modulated signal within a corresponding one of
multiple frequency channels. The method may further include
combining the frequency channels into a single electrical signal.
The method may include converting the electrical signal into an
optical signal for transmission to an optical node.
[0016] A method of providing dedicated bandwidth to each of
multiple subscriber destinations for delivering source information
over an HFC network is similar to the method describe above, and
includes modulating a multiplexed cell stream into an analog signal
in a frequency channel, converting the analog signal to an optical
signal, and transmitting the optical signal to the subscriber
destinations over the HFC network. The method may further include
receiving data packets at a distribution hub, decapsulating the
data packets to obtain IP packet data, and re-assembling the IP
packet data into IP packets. The method may further include
receiving an optical signal from a headend and converting the
optical signal into the data packets. The forwarding digital
information may include determining digital addresses associated
with the subscriber destinations. The method may include converting
the digital information into data packets, segmenting the data
packets in each data stream into packet segments, framing the
packet segments with frame headers, and encapsulating framed packet
segments into the data cells, where each data cell includes a cell
header. The method may further include transmitting the optical
signal to an optical node, converting, by the optical node, the
optical signal to an electrical signal, and transmitting the
electrical signal from the optical node to the subscriber
destinations via a coaxial cable.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] For a more complete understanding of the present invention,
reference is now made to the following description taken in
conjunction with the accompanying drawings in which like reference
numerals indicate like features and wherein:
[0018] FIG. 1 is a block diagram of a communication network
architecture according to an exemplary embodiment of the present
invention.
[0019] FIG. 2 is a simplified block diagram of an exemplary
embodiment of any of the distribution hubs of FIG. 1.
[0020] FIG. 3 is a functional block diagram of an exemplary packet
switch router implemented according to an embodiment of the present
invention.
[0021] FIG. 4 is a functional block diagram of an exemplary
embodiment of any of the channel interface modules of FIG. 3.
[0022] FIG. 5 is a flowchart diagram illustrating downstream cell
processing performed by the channel interface modules of FIG.
3.
[0023] FIG. 6A is a block diagram illustrating IP packet
decapsulation and cell encapsulation for downstream transmission by
the cell processing engine of FIG. 4.
[0024] FIG. 6B is a block diagram illustrating CCP and PAP header
agreement between successive CCP cells.
[0025] FIG. 7A is a block illustrating multiplexing of a physical
channel for handling multiple data streams, where each stream is
assigned a corresponding transport channel to formulate a
multiplexed cell stream.
[0026] FIG. 7B is a block diagram illustrating multiplexing of a
physical channel for handling multiple data streams, where some
streams are assigned multiple transport channels to formulate the
multiplexed cell stream.
[0027] FIG. 8 is a block diagram illustrating the main components
and summarizing operation of the cell processing engine of FIG.
4.
[0028] FIGS. 9A-9C illustrate the relationship between the
scrambling, encoding and the interleaving process performed by each
modulator 401 with the cell convergence process performed by the
cell processing engine of FIG. 4.
[0029] FIG. 10 is a simplified block diagram of exemplary customer
premises equipment (CPE) located at each subscriber destination
that tunes, decodes, and de-modulates source information from a
combined electrical signal.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0030] FIG. 1 is a block diagram of an exemplary communication
system 100 with an exemplary network architecture. One or more
sources 101 are coupled via appropriate communication links 102 to
deliver source information to a headend 103, which further
distributes the source information to one or more distribution hubs
105 via respective communication links 104. Each distribution hub
105 further distributes source information to one or more nodes 107
via communication links 106, where each node 107 in turn
distributes the source information to one or more subscriber
destinations 109 via subscriber medium links 108. In the embodiment
shown, bi-directional communication is supported in which
subscriber information from any one or more of the subscriber
destinations 109 is forwarded to a corresponding distribution hub
105. Depending upon the type of subscriber information and the
architecture implementation, the subscriber information may further
be forwarded by a distribution hub 105 to an appropriate source
101, either directly or via the headend 103.
[0031] It is noted that the headend 103, the distribution hubs 105
and the nodes 107 may generically be referred to as points of
distribution for source and subscriber information. Each point of
distribution supports a successively smaller geographic area. The
headend 103, for example, may support a relatively large geographic
area, such as an entire metropolitan area or the like, which is
further divided into smaller areas, each supported by a
distribution hub 105. The areas supported by each distribution hub
105 is further divided into smaller areas, such as neighborhoods
within the metropolitan area, each supported by a corresponding
node 107.
[0032] Many different types of sources 101 are contemplated, such
as one or more computer networks 111, one or more telephone
networks 113, one or more satellite communication systems 115, one
or more off-air antenna systems 116 (e.g. microwave tower), etc.
The computer networks 111 may include any type of local, wide area
or global computer networks, such as including the Internet or the
like. The telephone networks 113 may include the public switched
telephone network (PSTN). The satellite communication systems 115
and/or the antenna systems 116 may be employed for reception and
delivery of any type of information, such as television broadcast
content or the like. The headend 103 may also include video on
demand (VOD) equipment (not shown). Depending upon particular
configurations, any one or more of the sources 101 may be coupled
directly to one or more of the distribution hubs 105 in the
alternative or in addition to being coupled to the headend 103 as
illustrated by communication links 102'. For example, one or more
of the computer networks 111 and the telephone networks 113 are
shown coupled to a distribution hub 105 in addition or in the
alternative. The headend 103 includes appropriate equipment for
data transmission, such as, for example, internal servers,
firewalls, IP routers, signal combiners, channel re-mappers,
etc.
[0033] Each of the communication links (102, 102', 104, 106, 108)
may be any appropriate type of medium, such as electrical or fiber
optic cables or the like, or any combination of mediums, such as
including both electrical and optical media or multiple optical
media, etc. For example, in one embodiment, each of the
communication links 102 and 102' includes optical media for
communicating analog optical information, such as between the
headend 103 and a satellite communication system 115 or an antenna
system 116, and/or 1000Base-X Ethernet for communicating digital
data and information between the headend 103 and any computer or
telephone network 111, 113. Also, the communication links 106
comprise optical fibers or cables that are distributed between each
node 107 and a corresponding distribution hub 105. The network
architecture may employ a hybrid fiber coax (HFC) distribution
network in which the subscriber medium links 108 comprises coaxial
cables that are distributed from each node 107 to the respective
subscriber destinations 109. In this configuration, the nodes 107
are optical nodes for conversion between optical and electrical
formats. The communication links 104 may also comprise optical
links, such as, for example, SONET (Synchronous Optical Network)
links or the like. It is understood that any known or future
developed media is contemplated for each communication link. In an
HFC embodiment, for example, each node 107 receives an optical
signal from an upstream point of distribution, converts the optical
signal to a combined electrical signal and distributes the combined
electrical signal over a coaxial cable to each of several
subscriber destinations 109 of a corresponding geographic serving
area. Subscriber information is forwarded in electrical format and
combined at each node 107, which forwards a combined optical signal
upstream to a corresponding one of the distribution hubs 105 via
respective communication links 106.
[0034] Each subscriber destination 109 includes customer premises
equipment (CPE) 1001 (FIG. 10), such as set-top boxes or cable
modems or the like that tunes, decodes, and de-modulates source
information from the combined electrical signal addressed or
otherwise intended for the particular subscriber destination 109.
The CPE at each subscriber destination 109 includes a modulating
device or the like that encodes, modulates and up converts
subscriber information into RF signals. The upstream RF signals
from each of the subscriber destinations 109 are transmitted on a
subscriber medium 108 to a corresponding node 107. A separate
upstream channel of the upstream portion of the cable spectrum used
for upstream communications may be assigned to each of the
subscriber destinations 109 to prevent interference with downstream
communications. The upstream RF signals are provided to the node
107, which includes an upstream optical transceiver or the like
that converts the subscriber RF signals to an optical signal. For
example, laser in the node 107 may be used to convert the return
signal to an optical signal and send the optical return signal to
an optical receiver at the distribution hub 105 over another fiber
optic cable.
[0035] The source and subscriber information may include any
combination of video, audio or other data signals and the like,
which may be in any of many different formats. The source
information may originate as fixed- or variable-size frames,
packets or cells, such as Internet protocol (IP) packets, Ethernet
frames, Asynchronous Transfer Mode (ATM) cells, etc., as provided
to the distribution hubs 105. Any such type of digital information
in fixed- or variable-sized frames, packets or cells is referred to
herein as "packetized" data. The packetized data includes one or
more destination addresses or the like indicating any one or more
specific subscriber devices at the subscriber destinations 109. In
exemplary embodiments of the distribution hubs 105 as described
herein, the packetized data is converted and delivered to the
subscriber destinations 109 employing time-division multiplexing
(TDM) over broadband modulation. Such TDM over broadband modulation
enables the delivery of allocated, unshared and deterministic
bandwidth to the subscribers in the communication system 100. The
CPE at each subscriber destination 109 includes the appropriate
communication equipment to receive, demodulate and decode the TDM
over broadband information to deliver the original content to the
subscriber. Upstream subscriber information may be handled in a
similar manner, and will not be further described herein.
[0036] It is noted that many different modulating frequencies and
techniques are contemplated for both downstream and upstream
communications. Modulation techniques may include, for example,
Frequency Shift Keying (FSK), Quadrature Phase-Shift Keying (QPSK),
as well various types of Quadrature Amplitude Modulation (QAM),
such as QAM 16, QAM 64, QAM 256, etc., among other modulation
techniques. Also, each frequency or "physical" channel may have any
predetermined bandwidth, such as 1 MHz, 3 MHz, 6 MHz, 12 MHz, etc.
Each channel typically includes a separate downstream and upstream
channel separated in frequency, where the corresponding down and up
stream channels may have the same or different channel width.
Further, the modulation technique employed for each downstream
channel may be the same or different than the modulation technique
employed for each upstream channel.
[0037] In one embodiment, the communication system 100 is an HFC
system that supports analog television broadcast transmission in
which broadcast television channels are allocated to a particular
frequency range of the overall available RF cable television
spectrum (5 MHz-1 GHz). The remaining portion of the RF cable
television spectrum is utilized to assign data channels including
any combination of downstream and upstream channels. For example,
some HFC systems implement an extended sub-split frequency plan
with a return band, which extends from 5 to 42 MHz, and a forward
band, which extends from 52 to 750-860 MHz. It is understood that
the particular frequency ranges described herein are exemplary only
and that any frequency allocation scheme may be employed depending
upon the desired configuration. In one exemplary embodiment, the
entire forward band is segmented into 6 MHz channels according to
the channelization plan implemented by the particular HFC network
operator. For typical HFC plants supporting analog television
broadcasts, 80 analog channels are transmitted in the forward band
between 53 and 550 MHz. In such HFC networks, satellite signals and
local analog stations are mapped to 6 MHz broadcast channels within
the forward band at the headend 103. Each 6 MHz forward band
channel may contain an analog channel or multiple digital channels
that are MPEG encoded. Each 6 MHz channel is upconverted to a
frequency within the forward band according to the appropriate
channelization plan.
[0038] The return band (5-42 MHz) and the remaining forward band
spectrum, including frequency ranges 550 to 750-860 MHz, is
allocated to subscriber digital channels and/or data transmission
for dedicated bandwidth to each subscriber destination 109. For
example, the frequency range 550 to 860 MHz is allocated for
downstream channels and the frequency range 5 to 42 MHz is
allocated for upstream channels. The frequency range 42-54 MHz is
the location of a diplex filter that separates the downstream from
the upstream communications. Diplex filters allow for
bi-directional communication over the shared HFC fiber and coaxial
medium using Frequency Division Multiplexing (FDM). The basic
diplex filter consists of a high pass and a low pass filter in
parallel followed by an amplifier that are both driven from the
same source.
[0039] In alternative embodiments of the communication system 100,
such as an all-digital HFC system, a substantial portion or the
entire available spectrum is utilized to assign channels to each of
the subscribers. In an all-digital HFC network, for example, there
is no requirement for broadcast transmission of analog channels
over the same frequencies used to transmit broadcast channels using
off-air frequencies (i.e. Channel 2 at 54 MHz in the HRC frequency
plan). As a result, the filter frequency settings on the diplex
filter in an all-digital network may allow increased spectrum
allocation for upstream communications. For instance, mid-split and
high-split frequency plans, which are suitable for an all-digital
network, allocate the 5-86 MHz and 5-186 MHz ranges, respectively,
for upstream transmission. Consequently, all-digital networks allow
more upstream bandwidth for interactive services such as data over
cable services. In these all-digital embodiments, the relatively
large bandwidth otherwise consumed by television broadcast
information is available for channel assignments. This provides a
significant advantage since a very clean portion of the RF spectrum
(e.g., 50-300 MHz) may be employed for data communication. Each
user may be allocated a greater amount of bandwidth or a greater
number of subscribers may be served for each coaxial cable. A
different frequency spectrum split may be utilized to increase
upstream bandwidth availability, and enables a symmetrical
configuration with equal downstream and upstream bandwidth.
Embodiments with a smaller geographic serving area provide a
reduced noise node so that each subscriber destination 109 receives
a cleaner signal, typically without the need for amplification.
[0040] One significant benefit of the embodiments of the
communication system 100 described herein is the ability to deliver
allocated, unshared and deterministic bandwidth to individual
subscribers. Thus, data destined for a particular subscriber
destination 109 is assigned a specific and unshared bandwidth that
is available only to that subscriber. This provides the ability to
deliver time-dependent or isochronous type services to each
subscriber destination 109, such as video, voice over IP,
bi-directional audio content (e.g., a telephone connection), etc.,
that is not otherwise possible in a network in which data over
cable delivery methodologies that use contention- or
arbitration-based bandwidth allocation schemes are implemented.
Bandwidth allocation may be controlled by a bandwidth manager or
the like at each distribution hub 105. The bandwidth manager
allocates each subscriber destination 109 unshared and
deterministic bandwidth in both down and upstream directions.
[0041] FIG. 2 is a simplified block diagram of an exemplary
embodiment of any one or more of the distribution hubs 105 of FIG.
1. In the embodiment shown, the distribution hub 105 includes a
switch 201 that receives and forwards digital information, such as
data and content, between the upstream sources via the
communication link 104, such as the sources 101 and/or the headend
103, and one or more (N) packet switch routers (PSRs) 203. The
switch 201 and each PSR 203 may be configured to communicate via
optical media or the switch 201 may include optical to electrical
(O/E) conversion. In one embodiment, the switch 201 is an
Ethernet-type switch that forwards Ethernet packets. Each packet
includes source and destination addresses enabling the switch 201
to forward the packets from a source to the appropriate destination
in both upstream and downstream directions. In a more particular
embodiment, the switch 201 includes one or more switches each
operating according to 100Base-X or 1000Base-X Ethernet at a data
rate of 100 Mbps or 1 gigabit per second (Gbps), respectively. Each
PSR 203 is interfaced with the switch 201 via separate and
respective optical or electrical 100Base or 1000Base Ethernet
electrical or optical links 214. It is understood, however, that
the present invention is not limited to any particular
architecture, protocol or technology and that other network
technologies may be used, such as Asynchronous Transfer Mode (ATM)
technology or the like.
[0042] As further described below, each PSR 203 encodes, modulates
and up converts source digital information received from the switch
201 into one or more downstream channels, and forwards RF signals
to respective inputs of at least one of one or more RF electrical
to optical (E/O) combiners and transmitters 205. Each RF channel
has a predetermined frequency bandwidth, such as 6 MHz in a
standard United States configuration, and thus supports a
particular amount of data transmission depending upon the
modulation technique employed. In a particular embodiment employing
QAM-256 as the modulation technique, each 6 MHz physical channel
has a data throughput capacity of approximately 40 Mbps. It is
appreciated that alternative modulation techniques other than
QAM-256 may be employed. The PSR 203 may be implemented in a
modular and scalable format to combine multiple downstream channels
into at least one combined electrical signal distributed via a
single RF connector. Also, each PSR 203 may be implemented to
provide multiple combined electrical signals via corresponding RF
connectors, each supporting multiple downstream channels. Each
combiner/TX 205 combines the RF signals from one or more combined
electrical signals from one or more PSRs 203 into a single combined
optical signal that is transmitted via a fiber optic cable or the
like to a corresponding one of the nodes 107. It is noted that each
distribution hub 105 may transmit to one or more nodes 107, each
serving a different geographic serving area.
[0043] Upstream subscriber digital information is received by a
corresponding one of several RF optical to electrical (O/E)
receivers and splitters 207, which receives an optical signal with
combined subscriber information via an optical cable, converts the
combined optical signal to a combined subscriber electrical signal
and splits or duplicates and forwards the combined subscriber
electrical signal to corresponding one or more of the PSRs 203. It
is noted that the upstream signals are typically received over
diverse return paths from separate nodes. In the embodiments
described herein, the upstream signals are combined to a single
signal that is received by a common PSR input connector. As further
described below, each PSR 203 is tuned to one or more upstream
channels and extracts a corresponding return RF signal. Each PSR
203 demodulates and decodes the return RF signal into corresponding
subscriber data packets for each upstream channel. The subscriber
data packets are then forwarded to the switch 201 for processing
and/or forwarding as necessary. It is noted that although a
separate combiner/TX 205 and a separate splitter/RX 207 is shown
for each PSR 203, multiple combiner/TXs 205 and splitter/RXs 207
may be provided for a single PSR 203 or multiple PSRs 203 may use a
single combiner/TX 205 and/or a single splitter/RX 207 depending
upon particular configurations and data capabilities of the
respective devices.
[0044] The distribution hub 105 may include one or more local
content servers that convert or otherwise deliver data and content
between the distribution hub 105 and the subscriber destinations
109 and/or upstream sources, such as the sources 101 and/or the
headend 103. For example, the distribution hub 105 may include one
or more video servers 209 that communicate video content, one or
more computer network servers 211 that enable communication with
the internet and/or other computer networks, and one or more
telephone network servers 213 that enable communication with the
PSTN and/or other telephonic networks. Also, the distribution hub
105 may include one or more broadcast content servers 215 for
receiving and forwarding broadcast content and information, such as
television broadcast channels or the like. Such broadcast content
and information may be selectively delivered within individual
subscriber channels or collectively broadcast with the subscriber
channels as previously described. Each of the servers 209-215
represents one or more server computers and includes any additional
functionality as necessary or desired. For example, the video
servers 209 may incorporate one or more video functions including
video-on-demand (VOD) and may further include an MPEG (Moving
Pictures Experts Group) converter or the like that converts
broadcast video content from analog to digital or otherwise
transcodes video content from one digital form to another. The
telephone network servers 213 may include or otherwise incorporate
one or more telephone switches or the like. The illustrated servers
209-215 are exemplary only and other types of servers and content
are contemplated. Alternatively, the servers 209-215 may be
replaced by a generic data server for exchanging information with
the headend 103.
[0045] In one embodiment, broadcast content is received from an
upstream source via the communication link 104 and provided to an
O/E converter 217. The electrical broadcast content is then
provided to a splitter 219 and distributed to respective inputs of
one or more of the combiner/TXs 205. The broadcast content may be
in either analog or digital format. Each combiner/TX 205 is
configured to receive and combine the broadcast television
information with the source information forwarded within assigned
channels from one or more of the PSRs 203. In particular, each
combiner/TX 205 operates to overlay the broadcast content
information, such as television broadcast channels or the like,
with the digital subscriber channels to develop a combined optical
signal for downstream transmission. The CPE at each of one or more
of the subscriber destinations 109 is configured to receive, split
and forward the broadcast content information to an appropriate
subscriber device, such as a set top box or television or the like.
This embodiment of the communication system 100 is particularly
applicable to consumer-based networks in which it is desired that
cable television channels or the like be available directly from
the subscriber medium routed to the subscriber destinations 109
without the need for further conversion.
[0046] In an alternative embodiment, the electrical broadcast
content is delivered to the broadcast content server 215 via
alternative connection 221, where the broadcast content server 215
is coupled to one or more of the PSRs 203 via separate connections
223 in a similar manner as the other local content servers 209-213.
In this manner, the broadcast content and information is
selectively delivered to subscriber destinations 109 via
corresponding subscriber channels. This embodiment of the
communication system 100 conforms to the all-digital configuration
in which the entire available spectrum is available for digital
communications via the subscriber channels.
[0047] FIG. 3 is a functional block diagram of an exemplary PSR 203
implemented according to an embodiment of the present invention.
The PSR 203 is deployed at a point of targeted service insertion,
which is usually at one or more of the distribution hubs 105 in an
HFC configuration. Targeted services are those services intended
for a subset of the entire HFC network subscriber base, such as VOD
services or the like. Targeted services are contrasted with
broadcast services where a given signal that originates from an
upstream source, such as the headend 103, to potentially serve all
subscriber destinations 109 within the general serving area of the
communication system 100. Analog and digital audio and video
services are examples of broadcast services. It is noted that many
details specific to cable transport, such as MPEG over ATM over
SONET transport (e.g. digital television or VOD services), are not
shown in the interest of focusing on the elements within the cable
network that are central to packetized data transport.
[0048] The PSR 203 includes one or more network interface modules
(NIMs) 301, each configured to interface and terminate a particular
network communication architecture. As shown, NIM 301a is coupled
to the communication link 104 to enable communications with
upstream sources, such as any of the sources 101 and/or the headend
103, etc., either directly or via the switch 201. The NIM 301a, for
example, may include a physical interface, such as a Gigabit Media
Independent Interface (GMII) conversion device (not shown) that
converts between a 1000Base-X fiber optic connection. In the
Ethernet embodiment, the NIM 301a terminates the GMII with an IEEE
802.3 Gigabit Ethernet Media Access Control (MAC) entity and
exchanges Ethernet frames with the GMII conversion device. Another
NIM 301b is provided to interface one or more of the local content
servers 209-215 via the appropriate communication standard, such as
100 or 1000Base-T Ethernet connections or links 214 previously
described. The PSR 203 may be implemented in a scalable manner to
allow additional NIMs 301, each configured to interface a separate
network media, protocol or architecture. In general, the NIMs 301
provide network interfaces to a high-speed local, metro or wide
area networks (LANs, MANs, WANs, etc.)
[0049] Each NIM 301 includes a physical interface for network
connectivity and integrated IP forwarding engines that forward
traffic between a network interface port and a switch 303. The NIM
functionality also includes physical encoding and link layer
framing. The switch 303 is coupled to one or more channel interface
modules (CIMs) 305, where each CIM 305 interfaces a corresponding
combiner/TX 205. The switch 303 forwards downstream information
from the NIMs 301 to a selected one of the CIMs 305, and forwards
upstream information from the CIMs 305 to one or more of the NIMs
301. As described further below, each of the CIMs 305 adapts IP
packets for synchronous downstream transmission and extracts IP
packets from synchronous bit streams in the upstream direction.
Each of the CIMs 305 forwards downstream data to at least one
combiner/TX 205 and receives upstream data from at least one
splitter/RX 205. As described further below, for transmission in
the downstream direction, each CIM 305 performs packet
encapsulation, forwarding, broadband packet encapsulation,
channelization, encoding, modulation and additional RF functions.
For transmission in the upstream direction, each CIM 305 performs
similar and inverse functions.
[0050] Each CIM 305 supports multiple downstream physical channels
combined and upconverted to a common carrier signal provided to a
corresponding combiner/TX 205 via a single connector. In one
embodiment, for example, the CIM 305 provides 8 QAM-256 modulated 6
MHz channels, where the corresponding combiner/TX 205 combines the
outputs of one or more CIMs 305. In an exemplary embodiment of the
communication system 100 that supports television broadcast content
in the 54-550 MHz range, the output of each CIM 305 resides within
the 550-750 MHz or 550-860 MHz range. The combined physical
channels are typically contiguous. In a particular example, if the
QAM signals have carrier frequencies of 600, 606, 612, 618, 624,
630, 636, and 642 MHz, then the output of the CIM 305 occupies the
597-645 MHz spectrum. An adjacent CIM 305 may have carrier
frequencies of 648, 656, 662, 670, 678, 686, 694, and 702 MHz
occupying the 645-705 MHz spectrum. As a result, the corresponding
combiner/TX 205 combines the 51-537 MHz broadcast spectrum with the
597-645 MHz or 645-705 MHz output of one CIM 305 or the 597-705 MHz
outputs of the two adjacent CIMs 305. The resulting RF signal is
converted to an optical signal and transmitted to a corresponding
node 107 by an optical transmitter. It is noted that since each PSR
203 provides a targeted service with spectrum that is only unique
to a particular node 107 served by an optical transmitter
corresponding to a PSR output, the same frequencies may be used for
transmission across multiple outputs of each hub 105.
[0051] The switch 303 and its interface to the NIMs 301 and CIMs
305 may be implemented in accordance with any one of many different
configurations, where the present invention is not limited to any
specific configuration. In one exemplary embodiment, the switch 303
is implemented in accordance with the Common Switch Interface
(CSIX) specification, such as CSIX-L0, CSIX-L1, CSIX-L2, etc. The
switch 303 and each NIM 301 and CIM 305 communicate across a common
bus or cross-bar switch (not shown) or the like using CFrames in
accordance with the applicable CSIX specification.
[0052] The switch 303 executes IP routing algorithms and performs
system management and control functions, either internally or via a
separate IP routing block 307 and a separate management block 309.
The switch 303 distributes routing tables to IP forwarding engines
located on each NIM 301 and CIM 305 via the illustrated connections
or through a separate control bus or serial link or the like. The
switch 303 also incorporates switch fabric that provides
connectivity for traffic between the NIMs 301 and the CIMs 305. The
switch 303 may include 10/100 Base-T Ethernet and asynchronous
interfaces for management connectivity. In one embodiment, the
switch 303 includes a high-speed, synchronous, bi-directional,
serial crossbar switch that performs the centralized switching
function in the PSR 203. The switch 303 includes a fabric
controller that is responsible for scheduling and arbitration in
the switch fabric architecture. The fabric controller manages the
connections through the switching fabric using an appropriate
scheduling algorithm that is designed to maximize the number of
connections per switching cycle. Management functions may be
handled within the switch 303 or by another management module 307
as illustrated. Each of the NIMs 301 and CIMs 305 may be coupled to
the management module 307 via separate management connections (not
shown).
[0053] FIG. 4 is a functional block diagram of an exemplary
embodiment of a CIM 305. The CIM 305 forwards IP packets and
performs packet framing and channelization. In addition, the CIM
305 performs the associated digital signal and RF processing for
transmission over the network architecture. Each CIM 305 includes a
cell processing engine 405 that interfaces the switch 303 via a
switch interface 409. The cell processing engine 405 may include
supporting internal or external memory for table-lookups, queued
data payload buffer descriptors and data payload buffer storage.
Such memory may include any combination of read only memory (ROM)
or random access memory (RAM) devices. The cell processing engine
405 processes each packet transferred between the network interface
via the splitter/RX 207 and the switch interface 409. The cell
processing engine 405 functionality includes IP forwarding, link
layer framing and physical layer encoding for transmission to the
combiner/TX 205 or to switch interface 409 for transmission to the
switch 303. In addition, the cell processing engine 405 performs
physical and link layer framing.
[0054] The CIM 305 also includes multiple modulators (MOD) 401 and
multiple demodulators (DEMOD) 403 coupled to the cell processing
engine 405 to enable broadband modulated transmission of packetized
data. In one embodiment, the modulators 401 perform continuous-mode
randomization, error encoding, interleaving and 256-point QAM, for
data transmission via the network. The outputs from the modulators
401 are combined in the frequency domain by an RF transmitter
network 411, which provides a single combined output via a
corresponding transmitter RF link. Such analog RF processing
includes filtering, frequency combining and mixing. Likewise, the
demodulators 403 receive upstream information through a
corresponding splitter/RX 207 via an RF receiver network 413. The
RF receiver network 413 processes analog RF signals, where such
processing includes frequency tuning, filtering and mixing. The
demodulators 403 perform similar and inverse functions of the
modulators 401 and will not be further described. The number of
modulators 401 and demodulators 403 may be the same for symmetrical
embodiments, although the present invention contemplates any number
of transmitters and receivers depending upon the particular
architecture and configuration.
[0055] In the downstream direction, the cell processing engine 405
forwards an IP packet from the switch interface 409 to the
appropriate channel based on destination IP address. As described
further below, the cell processing engine 405 performs data link
layer encapsulation using a packet adaptation procedure (PAP) to
encapsulate IP packets into frames. The cell processing engine 405
adapts the frames for cell transport suited for encoding using a
cell convergence procedure (CCP). In the exemplary embodiment
shown, such encoding is according to the Reed-Solomon (RS) encoding
procedure. The cell processing engine 405 also performs time
division multiplexing of dedicated time slots within each physical
downstream channel. The cell processing engine 405 adapts IP
packets for synchronous transmission and extracts IP packets from
synchronous bit streams. Each subscriber channel is a
bi-directional data link layer communications channel between the
PSR 203 and the CPE of each subscriber destination 109 served by
the PSR 203.
[0056] In the illustrated embodiment, (204, 188) RS encoding for
188-byte cell transport is employed although other types of
encoding or other variations are contemplated, such as, for
example, (255,239) RS encoding. The PAP encapsulates each IP packet
prior to transmission by pre-pending a PAP header to the IP packet
to formulate PAP frames. The CCP adapts the resulting PAP frames
for RS payload insertion by dividing PAP frames into segments and
inserting a header to each segment. The CCP header is a pointer
offset field that indicates the location of the first byte of a PAP
header within the RS payload.
[0057] FIG. 5 is a flowchart diagram that summarizes downstream
packet processing performed by the CIM 305. The CIM 305 performs a
series of protocol functions upon the ingress frames, adapting IP
packets into synchronous bit-streams for transmission over a
corresponding channel. The general process illustrated is agnostic
relative to the type of packets or frames; such as Ethernet frames,
ATM cells, CSIX frames, etc. At a first block 501, an input frame
is received by the cell processing engine 405 via the switch
interface 409. The cell processing engine 405 performs IP packet
decapsulation and/or re-assembly at next block 503, where
particular processing depends upon the particular packet data
format. For example, multiple CSIX cells each having equivalent
payloads on the order of 100 bytes are first decapsulated to
retrieve the IP packet payloads, which are then re-assembled
together to formulate the original IP packet. A similar process may
be performed for Ethernet frames, although one or more IP packets
may be entirely encapsulated within a single Ethernet frame. In any
event, the resulting IP packets are forwarded to an appropriate
channel corresponding to the destination address indicated in an IP
header at next block 505. In one embodiment, the cell processing
engine includes separate channel processing modules or blocks are
for each channel. Alternatively, the cell processing engine 405
separates the channels within its memory.
[0058] For each channel, the cell processing engine 405 performs
packet framing at next block 507. Such framing processing includes
the PAP and CCP procedures to encapsulate the IP packets into
frames and to adapt the frames into cells for cell transport suited
for encoding. The resulting cells are then channelized by the cell
processing engine 405 at next block 508. Such channelization
implements TDM within predetermined or pre-assigned time slots as
further described below in accordance with dedicated data
throughput subscriber channels. The resulting channelized data
stream of cells is provided to a corresponding one of the
modulators 401. The cell processing engine 405 performs the same
process for each physical channel handled by a corresponding one of
the modulators 401. Each modulator 401 performs continuous-mode
randomization (block 509), error encoding (block 511), interleaving
(block 513) and modulation (block 515) for data transmission. These
functions are described more fully below.
[0059] At next block 517, the digital data output from each of the
modulators 401 are provided to the RF transmitter network 411 for
RF processing and transmission. In particular, the RF transmitter
network 411 maps the data into code words, converts the code words
into a waveform, and modulates the waveform to an Intermediate
Frequency (IF), such as between 30 MHz and 60 MHz. The IF signal is
then upconverted to any one of several 6 MHz channels within the
applicable frequency range (550-860 MHz for the consumer broadcast
television embodiment) by an up converter (not shown). In one
embodiment, two stages of up conversion are used to achieve desired
signal-to-noise levels. The upconverted signal is amplified and
equalized for transmission over the TX RF link. The RF transmitter
network 411 performs RF aggregation and provides the ability to
operate anywhere within the applicable downstream frequency range
based on software configuration. The RF transmitter network 411
outputs an RF signal that incorporates the combined information
from each of the modulators 401.
[0060] Although not further described herein, a similar and
opposite process is performed by the CIM 305 in the upstream
direction. The RF receiver network 413 includes an RF tuner and
down converter (not shown) that tunes to a corresponding 6 MHz
upstream frequency employing phase-lock-loop (PLL) techniques or
the like. The RF receiver network 413 selects RF channels in the
applicable frequency range (5-42 MHz for the consumer broadcast
television embodiment) used for upstream transmission. In one
embodiment, the RF receiver network 413 provides the ability to
operate anywhere within the applicable downstream frequency range
based on software configuration. The RF receiver network 413
further band pass filters and down converts the signal to the IF
for use by a demodulator (not shown) within each demodulator 403.
Each demodulator 403 demodulates a corresponding IF signal
employing a particular modulation scheme, such as QAM-64 or QAM-256
or the like and forwards the demodulated signal to a
decoder/descrambler (not shown). The descrambler descrambles the
resultant signal and decodes the data link encapsulated IP data
stream, such as using RS decoding or the like. The decoded cells
are forwarded to the cell processing engine 405, which performs
time division de-multiplexing of dedicated time windows
corresponding to upstream channel slots within each physical
channel. The cell processing engine 405 further performs an inverse
CCP and data link layer decapsulation of resultant IP frames using
an inverse PAP. The resulting IP packets are forwarded to the
switch 303 via the switch interface 409.
[0061] FIG. 6A is a block diagram illustrating IP packet
decapsulation and cell encapsulation for downstream transmission by
the cell processing engine 405. As further described below,
synchronous, byte-oriented processing utilizes packet and cell
headers to allow variable-length IP packets to be transported
across the network as a series of payload cells. These packet and
cell headers provide the destination with enough information to
reassemble the individual cells back into the original IP packets
to decode the message. The adaptation and convergence procedures,
described further below, also perform null packet generation and
added error protection. It is understood that although the present
invention is illustrated with IP packets, the present invention
applies to any type of digital information, including various types
of packetized information and data packets.
[0062] In one embodiment, an exemplary Ethernet encapsulated
protocol data unit (PDU) 601 is shown including a header 603, an IP
packet payload 605 and a Frame Check Sequence (FCS) 607 or the
like. The IP packet payload 605 is decapsulated from the Ethernet
frame 601 and forwarded to the appropriate channel within the cell
processing engine 405 corresponding to a destination IP address in
the header 603. The IP packet payload 605 becomes all or a portion
of an IP packet payload 615 of a PAP frame 616 further described
below. For an Ethernet embodiment, the header 603 may include an 8
byte preamble, a 6 byte destination address, a 6 byte source
address, a 2 byte length, an optional 8 byte Logical Link Control
Sub-Network Access Protocol (LLCSNAP) header, a 1,492 or 1,500 byte
IP packet payload and a 4 byte FCS. The preamble, addresses, length
and FCS fields form Ethernet framing. The Ethernet PDU 601 is a
maximum of 1526 bytes with 1,492 or 1,500 bytes of payload. Since
an IP packet may be up to 64 kilobytes (KB) in length, the IP
packet payload 605 may not include the entire contents of the
original IP packet. If so, multiple Ethernet PDUs are decapsulated
and the corresponding multiple IP packet payloads are re-assembled
into the original IP packet. This process is also known as
defragmentation.
[0063] To verify the validity of the IP packet payload for Ethernet
PDUs, the cell processing engine 405 utilizes both the FCS field
and an IP Checksum field within the IP packet payload 605. For
example, the FCS field is used to verify that the Ethernet PDU
traversed the network without incurring any bit errors. The FCS is
useful for detecting and protecting against synchronization errors
as well as transmission errors. The cell processing engine 405
performs a polynomial calculation on the bits of the Ethernet
Address, Length, LLCSNAP, and IP packet payload fields, and
compares the resulting 32-bit value with the value stored in the
FCS field. If the two values do not match, the cell processing
engine 405 discards the Ethernet PDU. To further verify the
validity of the embedded IP packet payload, the cell processing
engine 405 may use an error-detecting summing algorithm. If so, the
cell processing engine 405 considers the entire header 603 as a
sequence of 16-bit words, adding them up using ones complement
arithmetic and taking the ones complement of the result. If the
resulting checksum value does not equal the value stored in the IP
checksum field, the cell processing engine 405 assumes an error has
occurred during transmission and discards the Ethernet frame.
[0064] The decapsulation process is not limited to Ethernet and
similar or alternative decapsulation processes are contemplated.
For example, in an alternative embodiment, the cell processing
engine 405 receives a series of cells 609, such as CSIX type cells
(CFrames) or the like. Each cell 609 includes a header 611 or the
like and an IP payload 613 incorporating all or a portion of an
original IP packet. The IP payload 613 from one or more cells 609
is extracted and reassembled to form the IP Packet Payload 615 of
the PAP frame 616. Each header 611 includes similar type
information as the Ethernet PDU 601, where such information may be
utilized to perform error checking and/or correction in a similar
manner as described above. The header 611 also includes a
destination address or the like to facilitate IP forwarding in a
similar manner. A similar process may be employed for ATM cells or
any other type of packetized information utilized within the PSR
203.
[0065] The cell processing engine 405 performs the PAP to generate
the PAP frame 616, in which a PAP header 617 is appended to the
front of the IP packet payload 615. This process is referred to as
"framing" or encapsulation. In one embodiment, the PAP header 617
is 3 bytes long, including of a 1-byte control field 619 and a
2-byte length field 621. The control field 619 further includes a
packet type field 623 (4 bits), an extended header field 625 (1
bit), and a reserved field 627 (3 bits). The length field 621
specifies the number of bytes in the IP packet payload 615. The PAP
accomplishes inter-packet time fill by generating null packets with
the type field 623 set to null values or zero (0) bits. This
ensures synchronous transmission and helps eliminate the DC offset
of baseline wander. Furthermore, the PAP may provide additional
error correction by using simple parity on the PAP header 617 with
one of the reserved bits 627 of the control field 619.
[0066] In preparation for encoding, the cell processing engine 405
performs the CCP, which conducts a segmentation process by
accumulating the PAP-encapsulated IP bit-stream into "N" segments
629, 631, . . . , 633, where N is a positive integer and depends on
the size of the PAP header 617 and the IP packet payload 615. It is
noted that N may be one (1) in which an IP packet is below a
predetermined size and need not be divided further for insertion
into cells, as further described below. It is noted that although
the segments may be mostly equal in size, at least one segment is
usually smaller since the IP packets are variable in size and not
an exact multiple of a chosen segment size (e.g., a remainder
segment). A smaller segment is made equivalent in size by padding
it with zeroes or null values in preparation for the CCP. The PAP
header 617 is appended to or otherwise forms part of a first
segment 629. The CCP then attaches CCP headers 635 to the beginning
of each of the segments 629-633 to form corresponding CCP cells
641, 643, . . . , 645.
[0067] In one embodiment employing (204, 188) RS encoding, each CCP
cell is 188 bytes in length. The relative sizes of the CCP header
635 and the remaining segment may vary, where each segment may be
185 or 186 bytes in length. Each CCP header 635 includes a
synchronization value or "sync" byte 647 and a pointer offset field
649 (1 byte) that identifies the beginning of the next PAP header.
An optional control byte may be employed, but will not be further
described. If the pointer offset value in the pointer offset field
649 is within the appropriate range, the next IP packet begins in
the current cell. In a configuration in which the segments are 185
bytes in length, the appropriate range of the pointer offset value
is 0 to 185, inclusive, for (204, 188) RS encoding. If the pointer
offset value is equal to the maximum value of 204, thereby pointing
to the following cell's CCP header, the next IP packet does not
begin in the current CCP cell. Pointer offset values within the
remaining range (186 to 203 inclusive) are considered invalid or
are otherwise unused. It is noted that (255, 239) RS coding is also
contemplated, where the size of each CCP cell is 255 bytes so that
the relative sizes of the payloads and fields are changed
accordingly.
[0068] FIG. 6B is a block diagram illustrating CCP and PAP header
agreement between successive CCP cells 651 and 653. To ensure that
the CPE of the subscriber destination 109 can reliably reassemble
IP packets from a series of individual CCP cells, the CCP verifies
that the pointer offset values and the previous PAP header's length
field are in agreement. The first CCP cell 651 is followed by a
subsequent CCP cell 653, each including respective CCP headers 655
and 657. The CCP cells 651 and 653 are not necessarily consecutive,
in which case intermediate CCP cells include a CCP header with the
maximum value. The first CCP cell 651 includes a PAP header 659 and
a corresponding first portion of an IP packet 1. The CCP header 655
includes a pointer offset value indicating the position of the PAP
header 659 within the CCP cell 651. The PAP header 659 includes the
length field 621 defining the length of IP packet 1, and therefore
indicates the location of a subsequent PAP header 661 within the
subsequent CCP cell 653. The PAP header 661 is located at the
beginning of the next subsequent IP packet 2. The CCP header 657
includes a pointer offset value indicating the position of the PAP
header 661 within the CCP cell 653. Thus, the CCP verifies that the
CCP header 657 and the PAP header 659 are in agreement as to the
location of the next PAP header 661.
[0069] FIG. 7A is a block illustrating time division multiplexing
of a physical channel for handling multiple data streams, where
each stream is assigned a corresponding one of multiple transport
channels, and where each transport channel comprises a series of
corresponding time slots. In this example, eight different data
streams 701, individually labeled A-H, are each organized as a
series of CCP cells by the cell processing engine 405. Thus, data
stream A includes sequential CCP cells A1, A2, . . . , data stream
B includes sequential CCP cells B1, B2, . . . , etc. In the
embodiment shown, the cell processing engine 405 organizes or
channelizes the CCP cells into eight different transport channels
labeled 1-8, where each transport channel includes a corresponding
time slot of a predetermined number or group of repeating time
slots or cell groups 703 forming an outgoing multiplexed cell
stream. The data streams are handled in a round-robin manner by the
cell processing engine 405. The repeating cell groups 703 form a a
multiplexed cell stream generated by the cell processing engine
405. The resulting multiplexed cell stream is sent by the cell
processing engine 405 to a corresponding one of the modulators 401.
In this manner, data stream A, including CCP cells A1, A2, A3,
etc., is transmitted in transport channel 1. Likewise, data streams
B-H are each transmitted in transport channels 2-8, respectively.
In the exemplary embodiment, each data stream is thus allocated 1/8
of the total bandwidth of the multiplexed cell stream of the
physical channel. If the physical channel has a total data
throughput of approximately 40 Mbps assuming QAM-256 modulation,
then each transport channel effectively allocated approximately 5
Mbps. It is noted that the assignment of data streams to transport
channels is arbitrary, so that any data stream may be assigned to
any transport channel. Data stream A, for example, may be assigned
to any of the transport channels 2-8 rather than transport channel
1.
[0070] It is appreciated that the number of transport channels and
the number of data streams need not correspond or be equal. For
example, a larger number of data streams may be handled by the cell
processing engine 405 using a smaller cell group size by
subdividing a series of corresponding time slots into multiple
transport channels. For example, the transport channel 8 shown
populated with cells from the data stream H may be subdivided into
two different transport channels 8 and 9 for handling the data
stream H and an additional data stream I (not shown), respectively,
where the data streams H and I are allocated {fraction (1/16)} of
the total bandwidth. The respective transport channels 8 and 9
would each include every other 8.sup.th time slot in the
multiplexed data stream. It is noted, however, that the cell group
size and the number of transport channels could simply be changed
to nine (9), so that each of the nine data streams are allocated an
equal {fraction (1/9)} of the total bandwidth. It is not required
that the size of the transmission window be the same length of the
TDM frame. In this manner, it is understood that each transport
channel 1-8 need not be dedicated to or correspond with a
particular data stream, provided that the allocation is regular and
synchronous to maintain the essence of TDM.
[0071] FIG. 7B is a block diagram illustrating time division
multiplexing of a physical channel for handling multiple data
streams, where some streams are assigned multiple transport
channels. In this case, only four data streams are shown, labeled
A-D, whereas the same number of transport channels 1-8 is defined.
Also, each data stream is assigned to one or more specific
transport channels. Data stream A is arbitrarily assigned transport
channel 2 for 1/8 of the data throughput of the physical channel,
data stream B is assigned transport channels 1, 3, 4 and 6
resulting in 4 of the 8 channels or 1/2 of the data throughput,
data stream C is assigned transport two channels 7 and 8 for 1/4 of
the data throughput, and data stream D is assigned single transport
channel 5 for the final 1/8 of the data throughput. As illustrated,
the first cell group 703 is populated with cells B1-B4 and C1-C2,
the second cell group 703 is populated with cells B5-B8 and C3-C4,
and so on to maintain proper ordering of the cells for each data
stream. It is appreciated that the data streams are handled in a
weighted round-robin manner.
[0072] In general, given a cell group size of n, (where n=8 in FIG.
7A and corresponds to the number of time slots in each cell group),
each data stream corresponding to a subscriber destination 109 may
be assigned any one or more of the n transport channels to achieve
a corresponding bandwidth or data throughput. It is contemplated
that less than 1/n data throughput may be achieved by assigning a
data stream to less than one complete series of corresponding time
slots (thereby creating multiple transport channels for a given
stream of corresponding time slots), such as every other slot or
every fourth slot or the like in a given stream of corresponding
time slots. It is also possible to assign a subscriber destination
109 greater bandwidth than a given physical channel by allocating
at least part of a second channel. This later embodiment, however,
would require that the CPE at a subscriber destination 109 be
capable of tuning to more than one frequency channel. In one
embodiment, a lookup table or the like is used to create an
association between each logical channel associated with an IP
address or subscriber destination and one or more physical timeslot
channels. A time slot value may be used as an index to retrieve the
assigned logical channel from the lookup table. In this embodiment,
network management populates the lookup table prior to using the
physical channel. The cell processing engine 405 utilizes the
programmed values to allocate bandwidth to each data stream
received. It is noted, as further described below, that the input
data may generally be asynchronous and intermittent or "bursty". In
this manner, input data is not always available to populate the CCP
cells in each data stream. Partial cells may be padded with zeroes
or null values to formulate full cells. Also, the cell processing
engine 405 may generate null cells to fill in gaps of input data to
create a continuous multiplexed cell steam created by the
channelization process.
[0073] FIG. 8 is a block diagram illustrating main components and
summarizing operation of the cell processing engine 405 of FIG. 4.
Packets, frames or PDUs or the like are received by the cell
processing engine 405 as shown at 801, such as via the switch
interface 409. The cell processing engine 405 includes a packet
processor 803 that performs decapsulation and/or re-assembly,
resulting in a stream of IP packets as shown at 805. The term
"processor" as used herein does not necessarily denote a specific
processing device or unit, but simply denotes any logic, circuitry,
code, software, etc. that is configured to perform the functions
described. The IP packet stream is provided to a switch device 807
or the like that performs the forwarding function, resulting in
multiple streams of IP packets, as shown at 809. The multiple
streams of IP packets are provided to an encapsulator 808 that
further includes a PAP processor 810 and a CCP processor 812. The
PAP processor 810 performs the PAP on each data stream of IP
packets, adding a PAP header 617 to each packet, resulting in
corresponding streams of PAP frames 616 as shown at 811. The CCP
processor 812 then performs the CCP on each data stream of PAP
frames, segmenting the PAP frames into segments, and adding CCP
headers 635 to each segment in each stream, resulting in
corresponding streams of CCP cells 813 as shown at 815. The one or
more streams of CCP cells 813 are then provided to a channelizer
816, which performs the channelization function to form a
multiplexed stream of cells as shown as 817. The cell processing
engine 405 sends the multiplexed stream of cells to a corresponding
modulator 401 as shown at 819.
[0074] A memory 821, such as any combination of random access
memory (RAM) or read-only memory (ROM), may be incorporated within
or provided externally to the cell processing engine 405. The
memory 821 is a programmable device that stores values, variables,
data, or other parameters utilized by the cell processing engine
405 during operation. The memory 821 may store a lookup table (LUT)
823, that further includes time slot assignments for each data
stream. In a particular embodiment, the LUT 823 maps timeslots to
destination IP addresses corresponding to each data stream, where
the destination IP addresses each correspond to a subscriber
destination 109.
[0075] It is noted that only a subset of data for a single
frequency channel is shown, where it is understood that a greater
number of data streams may be processed for each channel, and that
multiple frequency channels may be included as desired. It is
further noted that the input data, in the form of packets, frames
or PDUs or the like, generally arrives asynchronously and
intermittently. One or more data streams may have no input data at
all. Also, the packetized data may have variable sizes. IP packets,
for example, are varied in size. In one embodiment, the cell
processing engine 405 outputs a continuous and synchronous steam of
multiplexed cells for each channel to a corresponding modulator.
Thus, some of the cells may be partially filled with data, where
the remaining portion of the cell is filled with zeroes or null
values. Also, during periods in which no input data is available
for a given data stream, the cell processing engine 405 outputs
null cells to the modulator. In this manner, one or more
asynchronous steams of downstream data is converted to a
synchronous streams of data cells that are modulated into
corresponding frequency channels.
[0076] FIGS. 9A-9C illustrate the relationship between the
scrambling, encoding and the interleaving process performed by each
modulator 401 with the framing process performed by the cell
processing engine 405. An exemplary CCP cell 905 is illustrated in
FIG. 9A including a sync byte 901 followed by a CCP payload 903. In
exemplary embodiments, most of the aspects of the digital coding
and modulation functionality of the modulators 401 is based on the
ITU J.83 Annex A recommendation (hereinafter "the ITU J.83
specification"). The ITU J.83 specification defines the framing
structure, channel coding and modulation for digital television,
audio and data signals distributed by cable networks possibly in
frequency-division multiplex (FDM). Such standard transmission
techniques may be employed in order to leverage existing
off-the-shelf technology. It is noted, however, that the present
invention contemplates any type of digital coding and modulation
functionality other than that described in the ITU J.83
specification. In one embodiment, the digital coding and modulation
functionality performed by each modulator 401 described herein is
based on the synchronization method provided in the ITU J.83
specification, which assumes an underlying MPEG framing format. As
a result, MPEG framing is not necessarily used in transmission.
Instead, the CCP sync byte is used to "spoof" an MPEG stream, which
allows the use of a standards-based synchronization method. By
using an industry-standard synchronization technique, off-the-shelf
components can be leveraged in the transmission system design.
[0077] The sync byte 901 is used as a synchronization mechanism for
the descrambler and decoder of the CPE at each subscriber
destination 109. In the embodiment shown, there are two valid sync
byte field values in which the second is a bit-wise inverted
version of the first. In one embodiment employing (204, 188) RS
encoding, the CCP cell 905 is 188 bytes so that the CCP payload is
187 bytes. In a more particular embodiment, the two valid sync byte
field values are 47.sub.HEX and B8.sub.HEX, where "HEX" denotes
hexadecimal notation. The sync byte of a first CCP cell in a group
of cells is bit-wise inverted from 47.sub.HEX to B8.sub.HEX to
provide an initialization signal for the descrambler. Each group
includes a designated number "m" of cells, although the present
invention is not limited to any particular group size. As an
example, the sync byte sequence for a succession of CCP cells for a
group size of eight includes seven cells with a sync byte of
47.sub.HEX followed by one cell with a sync byte of B8.sub.HEX.
[0078] After the channelization process, the scrambler or
randomizer process is applied resulting in a sequence of scrambled
CCP cells 909 as shown in FIG. 9B. Each scrambled cell 909 includes
a corresponding sync byte 901 and a scrambled CCP payload 907. The
randomizer process uses a predetermined polynomial for a
Pseudo-Random Binary Sequence (PRBS) generator (not shown). The
first sync byte 901, or each sync byte 1 of a repeating PRBS, is
inverted as illustrated by an overstrike. The first bit at the
output of the PRBS generator is applied to the first bit of the
first byte following the inverted MPEG-2 sync byte. To aid other
synchronization functions, during the sync bytes of the subsequent
transport packets, the PRBS generation continues, but its output is
disabled, leaving these bytes unscrambled. As a result, the period
of the PRBS sequence is 1503 bytes for 188-byte CCP cells. The
randomization process is also active when the modulator input bit
stream is non-existent, or when it is non-compliant with the
framing format. This is to avoid the emission of an un-modulated
carrier from the modulator.
[0079] Following the scrambling or randomizer process, each
scrambled CCP cell 909 is encoded into a codeword 913 as
illustrated in FIG. 9C. The codewords are referred to as RS
codewords when RS encoding is employed. The sync byte 901 is
inverted for a first cell in a PRBS series, whereas the remaining
sync bytes, denoted sync "x" where "x" varies from 2 to m, are not
inverted. The scrambled CCP payload incorporates an error detection
and correction (EDC) data 911 generated by the encoding process to
provide Forward Error Correction (FEC). The RS encoding is a
non-binary block coding scheme that corrects random bit and short
burst errors caused by noise during transmission. RS encoding uses
redundancy in a highly efficient manner, expanding each scrambled
cell 909 by adding redundant data or symbols. It is noted that the
EDC data 911 is not necessarily a separate field but may be
intermingled with the CCP payload. For (204,188) RS encoding, the
EDC field includes 16 parity or EDC bytes to achieve a (204, 188,
8) RS codeword. The EDC data for (204, 188, 8) RS encoding can
correct 8 erroneous bytes per RS codeword. It is noted that the
encoder also encodes the sync byte 901, where each sync byte 1 is
inverted as indicated by an overstrike. A predetermined code
generator polynomial and field generator polynomial are employed
for the RS encoding process. It is noted that a shortened RS
codeword may be implemented by appending 51 bytes, all set to zero,
before the information bytes at the input of a (255, 239) RS
encoder. After the coding procedure, the appended bytes are
discarded.
[0080] Following the encoding process, a convolutional interleaving
scheme is applied resulting in interleaved frames (not shown). In
one embodiment, the resulting interleaved frames are composed of
overlapping error-protected packets that are delimited by sync
bytes to preserve a periodicity of 204 bytes. The frames may be
interleaved in accordance with the ITU J.83 specification and will
not be further described. The interleaved frames are then
modulated, such as according to QAM-256 modulation as provided in
the ITU J.83 specification. The QAM process adapts the synchronous,
scrambled bit-stream for transmission over a channel as RF output.
The QAM process blocks together bits from the data stream and then
maps them into codewords using either Gray-codes or differential
codes. The QAM process then converts the resulting digital
codewords into an analog waveform based on a constellation diagram
of combinations of amplitudes and phases, where each unique bit
sequence corresponds to a point in the constellation.
[0081] It is noted that each modulator 401 receives CCP cells from
the cell processing engine 405 having a particular size whereas the
encoding process generates larger sized codewords. In this manner,
the timing differential between the cell processing engine 405 and
each modulator 401 is handled using any one of several optional
methods. In a first embodiment, the cell processing engine 405 adds
a time delay to each CCP cell equivalent to transmission of the
size differential. For example, in an exemplary embodiment, the CCP
cells are 188 bytes whereas the codewords are 204 bytes in length,
so that the cell processing engine 405 adds a time delay
differential equivalent to 16 bytes.
[0082] It is appreciated that each downstream channel handled by
each modulator 401 has a predetermined frequency bandwidth and a
corresponding data throughput. The protocol described herein has
provisions to further subdivide the physical medium into multiple
discrete channels using TDM. In one embodiment, such partitioning
is performed on a RS codeword basis, using the MPEG-2 sync field to
uniquely identify each of the multiple transport channels, in which
each transport channel includes a dedicated series of time slots
that are each sufficient to transmit one codeword. In this manner,
multiple and separate transport channels each share a common
physical channel. The entire physical channel may be used to
transport information to a single destination, such as one
subscriber destination 109, so that all of the transport channels
are assigned to the same subscriber. Alternatively, each transport
channel may be assigned to different subscriber destinations 109,
so that multiple subscribers share a physical channel. However,
since each subscriber destination 109 is assigned at least one
dedicated transport channel, each subscriber destination 109 is
provided dedicated and unshared bandwidth.
[0083] In a more specific embodiment, each CIM 305 transmits data
during dedicated transport channels over 6 MHz frequency channels,
where each transport channel includes a series of time slots. A
time slot is defined as the time required to transmit a 204 byte
Reed-Solomon codeword using QAM-256 at a symbol rate of 5.360537
Msym/sec or approximately 38 microseconds (.mu.sec). Each 6 MHz QAM
channel corresponds to a certain number of transport channels, such
as 8 transport channels, which are served in a round-robin manner.
During each time slot, a 204-byte Reed-Solomon payload is
transmitted to an error detection encoder for (204, 188) RS
encoding and consequential QAM transmission. Each connection can
receive from 1 to 8 transport channels, which need not be
contiguous. Consequently, bandwidth is allocated to each channel in
5.360537 Mbps increments up to 42.884296 Mbps. It is noted that
each subscriber destination 109 served by a frequency channel
maintains synchronicity with the transmitting CIM 305. The CPE at
each subscriber destination 109 extracts data only from its
assigned transport channel during the corresponding time
slot(s).
[0084] FIG. 10 is a simplified block diagram of exemplary CPE 1001
located at each subscriber destination 109, such as set-top boxes
or cable modems or the like that tunes, decodes, and de-modulates
source information from the combined electrical signal addressed or
otherwise intended for the particular subscriber destination 109.
The CPE 1001 may include a splitter 1003 coupled to a subscriber
medium link 108 for extracting broadcast content such as analog
television broadcast transmissions if transmitted. The remaining RF
spectrum dedicated to subscriber channels is provided to receiver
logic 1004 to extract source information. The splitter 1003 may not
be included in an all-digital configuration. The receiver logic
1004 includes an RF tuner 1005 that is tuned to a corresponding
physical channel transmitted by a corresponding CIM 305 of a PSR
203. For example, the RF tuner 1005 tunes to a corresponding 6 MHz
channel to which it is assigned. The filtered channel signal is
provided to a demodulator 1007, which generally performs the
inverse modulation procedure performed by a corresponding modulator
401, such as according to QAM-256 or the like. The demodulated
digital signal is then provided to a channel filter 1009 that
detects the sync bytes in the data stream and extracts one or more
digital codewords within each group of data that corresponds to its
assigned transport channels. Although the channel filtering
function may be performed later in the receiver process, early
filtering may simplify the subsequent portions of the receiver
logic 1004.
[0085] The filtered digital signal is provided to a decoder 1011,
which performs the inverse interleaving and encoding process
performed by a corresponding modulator 401, such as according to
the Reed Solomon encoding process previously described. The decoded
data is then provided to a descrambler 1013 to reverse the
randomization process. The resulting CCP cells are then provided to
CCP and PAP decapsulation logic 1015, which re-assembles the
original IP packets provided to the corresponding PSR 203. The IP
packets are then forwarded by IP forwarding logic 1017 to an
appropriate subscriber device as indicated by a destination
address. For transmission, IP packets from one or more subscriber
devices are forwarded by the IP forwarding logic 1017 to
transmitter logic 1019 and asserted onto the subscriber medium link
108. The upstream transmission process is not further described as
beyond the scope of the present disclosure.
[0086] Although various embodiments of the present invention have
been described in detail, it should be understood that various
changes, substitutions and alterations can be made hereto without
departing from the spirit and scope of the invention as described
by the appended claims.
* * * * *